The main objective of this five year study is to determine the low-dose-response range of Norwalk virus (NV) infectivity, and to define a dose-response curve for Snow Mountain virus. Snow Mountain virus is a related virus causing the same acute gastroenteritis as Norwalk. Norwalk virus is the leading cause of epidemic viral gastroenteritis in older children and adults int eh United States. It is typically found in drinking water, recreational water, ice, shellfish, various food items and environmental contaminations. It is likely that a significant portion of "acute gastrointestinal illness of unknown etiology" episodes are related to Norwalk virus, and that Norwalk and related viruses may be responsible for up to 23% of the waterborne outbreaks of acute gastroenteritis.